A warm Neptune with a primitive atmosphere

An article published in the journal “Science” describes a research on the exoplanet HAT-P-26b, a warm Neptune, meaning a planet of size similar to Neptune that orbits near its star HAT-P-26. A team of researchers from NASA’s Goddard Space Flight Center and the British University of Exeter used the Hubble and Spitzer space telescopes to discover what is called a primitive atmosphere for HAT-P-26b despite its star being old.

In the last two decades astronomers discovered many planets of the hot Jupiter type, meaning gas giant that orbit near their stars, with the consequence that they receive from it an amount of energy that make them even very hot. They found less smaller gas more like Uranus and Neptune, close to their stars but HAT-P-26b, one of these warm Neptunes, turned out to be really interesting.

The star HAT-P-26 is about 437 light-years away from the Earth, with a mass a bit lower than the Sun’s and an age that is about twice the Sun’s. The discovery of the planet HAT-P-26b was announced at the beginning of 2011 and immediately it was noted that it was wrapped in a hydrogen and helium atmosphere. This was surprising because in the solar system Jupiter and Saturn are composed almost entirely of those elements but Neptune and Uranus are richer in heavier elements. The mass fraction of these elements is called in jargon metallicity.

According to current models, the difference in composition between the gas planets is due to the fact that Uranus and Neptune formed in an outer region of the protoplanetary disk of gas, dust and debris around the newborn Sun. For this reason, they were bombarded by icy debris rich in heavier elements. Jupiter and Saturn formed in a nearer and warmer region where there were less icy debris, so they were hit by very few debris containing heavier elements.

HAT-P-26b is a gas planet far closer to its star than Jupiter with a mass similar to that of Neptune but with a composition much more similar to that of Jupiter. Gas planets form at some distance from their star, where there’s more gas and dust because they are not swallowed by the star, but can later get closer to it.

The position where gas planets formed can be recognized by their composition but in the case of HAT-P-26b it seems that it formed not too far from its current position. An alternative hypothesis is that it formed later during the development of the HAT-P-26 system when there were no more debris containing elements heavier than hydrogen and helium.

The researchers also found traces of water and clouds in HAT-P-26b in the best measurement of the presence of water for an exoplanet of that size. That measurement also helped estimate this planet’s metallicity. They’re all information that can help to understand the origin and evolution of HAT-P-26b, which suggest a greater diversity than expected in this type of planet.

This research was possible thanks to the combined use of the Hubble and Spitzer space telescopes, which observed HAT-P-26b when it passed in front of its star. The planet’s atmosphere filters starlight in a way that depends on its composition, allowing it to be analyzed. The researchers used Hubble’s data from four transits and Spitzer’s data from two transits. It’s a type of study still in development that is giving really interesting results.